1. Technical Field
The present invention relates to a method of determining reliability of a received signal, a method of calculating a code phase error, and an apparatus for determining reliability of a received signal.
2. Related Art
As a positioning system using satellite signals, a GPS (Global Positioning System) is widely known, and is used in a receiving apparatus that is built into a portable phone or car navigation. The GPS performs a position calculation for obtaining position coordinates of the receiving apparatus and timepiece error, based on information on positions of a plurality of GPS satellites and pseudo distances from the respective GPS satellites to the receiving apparatus.
One of the main causes of error occurrence in calculating a position using a satellite signal may be a multipath. An environment where a multipath occurs is called a “multipath environment”. The multipath environment is an environment where indirect wave signals, such as a reflected wave that is reflected from a building or the ground, a transmitted wave that is transmitted through an obstacle, a diffracted wave that is diffracted by an obstacle, and the like, are received as a multipath signal in superimposition with a direct wave signal from a source of the satellite signal (in the case of a GPS, a GPS satellite). In the multipath environment, the indirect wave signal becomes an error signal which makes decoding difficult, or a false code phase is calculated to cause an error that occurs during position calculation to be increased.
In order to solve the problem of the error that is caused by the multipath, for example, JP-A-2008-170214 discloses a technique of determining a positioning satellite that has a high possibility of being affected by the multipath as an inadequate positioning satellite and performing position calculation except for the inadequate positioning satellite.
One method for a multipath countermeasure may be a method that uses captured satellite signals in performing the position calculation to the exclusion of the multipath signal. This is because using the multipath signal in performing the position calculation may deteriorate the accuracy of position calculation. However, in a receiving environment such as an urban canyon environment where there are neighboring high-rise buildings, most of the received satellite signals may be the multipath signals. In this case, if all the multipath signals are excluded, the number of satellites that are required for the position calculation is unable to be secured. Accordingly, there is a demand for using satellite signals having high reliability, which are permissible for use in the position calculation, in performing the position calculation as much as possible.